A thick ass and legs. A reference to "bottom end power". Usually used in reference to a girl who is not fat, but has a bigger ass and thighs. Can be good or bad, depending on the user's preference for a thicker ass and thighs.

Damn, that bitch has some horsepower! Mmmm Mmmm!

or

Her face is hella pretty and she's got some nice abs, but she's runnin' too much horsepower for my taste.